1. Technical Field
The present invention relates to a terahertz radiating device and a method for fabricating the same. More particularly, the present invention relates to a terahertz radiating device capable of providing an electromagnetic wave signal source of a terahertz band using a transistor and a method for fabricating the same.
2. Description of the Related Art
The terahertz band is arranged between the RF band, microwave band and millimeter wave band, in which electronics technology is applied, and the frequency band of light in which photonics technology is applied. Research has been actively conducted on technology for generating/controlling/detecting electromagnetic waves of the terahertz as this technology is expected to be used for medical diagnostic technology, national security and short-range wireless communication employing extremely high frequencies.
However, the terahertz band, which is called the terahertz gap, still remains an unexplored area since it is not easy to apply the electronics technology and the photonics technology in the terahertz band.
In particular, the potential of the terahertz band has not been fully harnessed due to technical challenges of unit device technology for implementing a terahertz system. Specifically, signal source device technology for generating electromagnetic waves in the terahertz band is the biggest challenge. Challenges to be addressed to harness the full potential of the terahertz band includes not only generating electromagnetic waves in the terahertz band but also enabling a signal source device to operate at room temperature rather than at an extremely low temperature, enabling the device to operate at low voltage rather than at high voltage, designing a compact and lightweight device, and implementing electrical control of the device. Signal source devices of the terahertz band that have been introduced include a photoconductive antenna, quantum cascade lasers, a resonant tunneling diode, and an optical rectification device (the term “device” or “radiating device”, as used herein, preferably refers to a signal source device for the terahertz band). However, none of the aforementioned devices have met the requirements listed above.
Under these circumstances, plasma wave transistor technology has been introduced, which is a kind of signal source device technology for causing oscillation in the terahertz band based on instability of plasma in the two-dimensional electromagnetic field in a transistor. This technology employs gradual amplification of the plasma mode in the two-dimensional electromagnetic field which occurs when electrical boundary conditions are unequally applied to the source electrode (hereinafter, referred to as source) and drain electrode (hereafter, referred to as drain) of the transistor. Once the plasma mode reaches a certain magnitude through amplification, the amplified plasma mode remains in steady state in the transistor channel without increase in amplitude. In addition, the transfer speed of plasma is greater than or equal to 10 times the speed of an electron in the two-dimensional electromagnetic field, and accordingly the plasma mode enables operation in the terahertz band, overcoming the limit of the switching speed that depends on conventional electron transport.
Electromagnetic radiation using the plasma wave transistor technology in the terahertz band has been experimentally observed in a high electron mobility transistor (HEMT) employing compounds in Groups III-V at a low temperature and room temperature. The output power of electromagnetic waves achieved in the terahertz band with this configuration is only in the range of microwatts. The output power is one of the most important performance indicators for signal source devices. Herein, it should be noted that efficient transmission of energy stored in the plasma mode formed in the signal source device is important and a ratio of the power of an electromagnetic wave generated in the terahertz band to the utilized power depends on efficiency of the applied transmission technique.
As described above, the plasma wave transistor technology based on plasma instability utilizes non-uniformity of electrical boundary conditions of the source and drain of a transistor. Particularly, when the drain is electrically shorted in a high-frequency region, oscillation occurs easily. In this case, oscillation of a drain voltage is observed but outputting power through the drain is not possible. Accordingly, research has been conventionally conducted on outputting power through the gate electrode (hereinafter, referred to as gate) rather than through the drain.
For example, efforts have been made to fabricate a gate in a grating structure to efficiently convert the plasma mode in the transistor channel to an electromagnetic wave. A typical example is disclosed in U.S. Pat. No. 8,304,812 (“Terahertz wave radiating element”) granted to Panasonic Corporation. However, the output power of electromagnetic wave radiation obtained in the terahertz band according to the aforementioned patent document is insubstantial. It is thought that this is because combination between the gate electrode fabricated in the grating structure and the plasma mode is not sufficiently strong.